A computation technique is presented to analyse the electromagnetic response of High Temperature Superconducting (HTS) film purity using a slotline. The complex conductivity of HTS films for three different material purity is modeled using Coffey-Clem (CC) unified theory. By using the surface impedance calculated by CC model, the dyadic Green's functions in Spectral Domain Method (SDM) are formulated for the slotline. The Galerkin's procedure is employed for the computation of the propagation parameters when HTS slotlines of different purities are guiding a fixed frequency microwave signal. Changes in the propagation parameters are analyzed for the variation of the film thickness at different working temperatures. The paper presents a modeling method for studying the microwave response of the HTS film purity.